The single best source of vitamin D is sunlight. Your skin produces vitamin D naturally when exposed to UVB rays, and just 5 to 30 minutes of midday sun on bare arms can generate far more than any food provides. But sunlight isn’t always practical or sufficient, so the best overall strategy combines brief sun exposure with vitamin D-rich foods and, when needed, fortified products.
How Your Skin Makes Vitamin D
Your skin contains a cholesterol compound that UVB radiation (wavelengths between 295 and 315 nanometers) converts into a precursor of vitamin D3. That precursor then enters your bloodstream and goes through two chemical conversions, first in the liver and then in the kidneys, to become the active hormone your body uses.
This process is remarkably efficient. A fair-skinned person can produce thousands of IU of vitamin D in under 10 minutes of summer sun, while most foods deliver only a few hundred IU per serving. The challenge is that several factors reduce how much vitamin D your skin actually makes:
- Skin tone: People with darker skin need 3 to 6 times longer sun exposure to produce the same amount of vitamin D as people with fair skin.
- Latitude and season: During winter months at higher latitudes, UVB rays are too weak for meaningful vitamin D production. In southern cities like Sydney, fair-skinned people need about 10 minutes of winter midday sun. In places like Melbourne or further from the equator, that jumps to 25 minutes or more.
- Sunscreen: SPF 15 filters out 93% of UVB rays, and SPF 30 blocks 97%. In practice, though, most people don’t apply sunscreen thickly or evenly enough to block all synthesis. Brief unprotected exposure before applying sunscreen is one common approach.
The practical takeaway: aim for short periods of midday sun on your arms or legs a few days per week, adjusting for your skin tone and location. This alone can keep many people in the optimal range.
Best Food Sources of Vitamin D
Very few foods contain vitamin D naturally, which is why deficiency is so common in people who get little sun. The richest dietary sources are fatty fish.
Spanish mackerel delivers about 248 IU per 3-ounce serving when raw. Wild-caught salmon is typically even higher, often providing 400 to 600 IU per serving depending on the species and whether it’s wild or farmed. Farmed salmon generally contains less vitamin D than wild-caught. Other good options include sardines, herring, and canned tuna, all of which contribute meaningful amounts.
Egg yolks provide a small but consistent dose, typically around 40 IU per yolk. Cod liver oil is an outlier in the other direction, packing over 1,000 IU per tablespoon, making it the most concentrated food source available. Beef liver offers modest amounts as well.
Since vitamin D is fat-soluble, eating these foods as part of a meal that includes some dietary fat helps your intestines absorb it more effectively. This happens naturally with fatty fish, but if you’re taking a supplement or eating fortified cereal, pairing it with a source of fat improves uptake.
Mushrooms: The Plant-Based Exception
Mushrooms are the only significant plant-based source of vitamin D, but there’s a catch. Most commercially grown mushrooms are raised in the dark and contain almost none. Untreated portabella mushrooms, for example, have only about 11 IU per 100 grams.
That changes dramatically with UV light exposure. When portabella mushrooms are treated with UV light for just 15 to 20 seconds, their vitamin D2 content jumps to around 446 IU per 100 grams. Some specialty mushrooms go even higher. Maitake mushrooms grown under proprietary UV-light methods have been measured at over 2,200 IU per 100 grams.
Look for packaging that says “UV-treated” or “high in vitamin D.” You can also place store-bought mushrooms gill-side up in direct sunlight for 15 to 30 minutes before eating them. They’ll convert some of their natural compounds into vitamin D2, the same form your skin would make from a plant sterol. It’s less potent than the D3 form found in animal foods and sunlight, but it still raises blood levels.
Fortified Foods and How They Compare
Because natural food sources of vitamin D are limited, fortification fills a significant gap for most people. Nearly all cow’s milk sold in the U.S. is voluntarily fortified with about 120 IU per cup. Plant-based milks made from soy, almond, or oats are typically fortified to similar levels, ranging from 100 to 144 IU per cup depending on the brand. Some brands of orange juice are also fortified, though amounts vary and aren’t standardized.
Fortified breakfast cereals contribute another 40 to 100 IU per serving in many cases. These numbers are modest on their own, but they add up across a full day of eating. Someone who drinks two cups of fortified milk and has a serving of fortified cereal is getting roughly 300 to 400 IU from fortification alone, which covers a meaningful portion of daily needs.
How Much Vitamin D You Actually Need
Most adults need 600 IU (15 mcg) of vitamin D per day, while adults over 70 need 800 IU (20 mcg). Infants need 400 IU daily. These are the recommended dietary allowances set by the National Institutes of Health, and they assume minimal sun exposure.
Blood levels tell a more precise story. Optimal vitamin D levels fall between 20 and 50 ng/mL on a standard blood test. Levels below 10 ng/mL indicate severe deficiency, while 10 to 19 ng/mL is considered mild to moderate deficiency. On the other end, levels above 50 ng/mL increase the risk of excess calcium in urine, and anything above 80 ng/mL can become toxic.
If you eat fatty fish a couple of times per week, drink fortified milk, and get some regular sun exposure, you’re likely meeting your needs without a supplement. But if you live at a northern latitude, have dark skin, spend most of your time indoors, or follow a vegan diet, a blood test is worth getting. Many people in these groups fall below 20 ng/mL without realizing it, and a daily supplement of 1,000 to 2,000 IU can bring levels back to the optimal range relatively quickly.
Putting It All Together
No single source of vitamin D works perfectly for everyone, which is why the most reliable approach layers multiple sources. Sunlight is the most powerful producer, but it’s inconsistent across seasons, latitudes, and skin tones. Fatty fish is the best dietary source by a wide margin, with wild salmon and mackerel delivering several hundred IU per serving. Fortified milk and plant milks provide a steady baseline. UV-treated mushrooms offer a surprisingly strong option for people avoiding animal products.
The practical ranking, from most to least vitamin D per “dose,” looks like this:
- Cod liver oil: over 1,000 IU per tablespoon
- Midday sun exposure: thousands of IU in minutes, depending on skin tone and season
- UV-treated maitake mushrooms: up to 2,200 IU per 100 grams
- Wild salmon: 400 to 600 IU per 3-ounce serving
- UV-treated portabella mushrooms: about 446 IU per 100 grams
- Spanish mackerel: about 248 IU per 3-ounce serving
- Fortified cow’s milk or plant milk: 100 to 144 IU per cup
- Egg yolks: about 40 IU each
If you’re getting some combination of these regularly, you’re covering one of the most common nutritional gaps in the modern diet.